1. Field of the Invention
This invention relates to an image forming apparatus such as an electrophotographic copying machine or an electrophotographic printer, and more particularly to a cleanerless image forming apparatus.
2. Description of Related Art
FIG. 12 of the accompanying drawings schematically shows the construction of an example of a cleanerless electrophotographic image forming apparatus as an example of the prior art.
The reference numeral 1 designates an electrophotographic photosensitive member as an image bearing member which is usually made into a drum shape (hereinafter referred to as the photosensitive drum) and is rotatively driven at a predetermined peripheral speed in the direction of arrow. This photosensitive drum 1 is uniformly charged by a primary charging device 3.
In the present example, the primary charging device 3 is a contact charging device using a charging roller (electrically conductive roller) as a contact charging member. The reference character a denotes a charging nip part. A predetermined superimposed voltage comprising a DC voltage and an AC voltage is applied from a voltage source (power supply) 6 to the charging roller 3, whereby the peripheral surface of the photosensitive drum 1 is uniformly charged to a predetermined polarity and potential.
Next, the application of light is effected from a exposure apparatus 4 onto the photosensitive drum 1 at an exposure position b correspondingly to image information inputted from an external apparatus to thereby form an electrostatic latent image.
This electrostatic latent image on the photosensitive drum 1 is visualized as a toner image in the developing region C of a developing apparatus 2 by a toner T having the same triboelectrification polarity as the applied voltage to the primary charging device 3.
In the present example, the developing apparatus 2 is a magnetic single-component developing apparatus. This developing apparatus 2 feeds the toner T in a toner container by a toner feeding member 23, rotates a developer bearing member (hereinafter referred to as a developing roller) 21 containing a stationary magnet therein and also forms a toner layer given triboelectrification charges on the surface of the developing roller 21 by a developer regulating member (hereinafter referred to as the developing blade) 22 for regulating the toner on the surface of the developing roller. The photosensitive drum 1 and the developing roller 21 are rotated in a forward direction in an opposed part as indicated by arrow in FIG. 12, and a predetermined gap is provided between the two.
A superimposed voltage comprising a DC voltage and an AC voltage is applied from a voltage source 8 to the developing roller 21, and the toner on the developing roller 21 given the charges is shifted to the photosensitive drum 1 in conformity with the aforementioned electrostatic latent image to thereby form a toner image and visualize the electrostatic latent image.
In synchronism with the forming of the toner image, paper (transfer material) P as a transferred member (recording medium) is transported from a cassette by transporting means comprising a pickup roller 10, and the toner image is transferred to the transfer material) P in a transferring nip part d by a transfer charging device (transfer roller) 5. The transfer material P is separated from the photosensitive drum 1, and subsequently is transported to a fixing apparatus 9, whereby the toner image is fixed as a permanent image.
Also, any toner T not transferred by the transfer charging device 5 but remaining on the photosensitive drum 1 is collected by the magnetic single-component developing apparatus 2 during the developing after the next process, i.e., in the case that the photosensitive drum 1 is continuedly charged, and is exposed to thereby form a latent image, and a toner image is formed in the magnetic single-component developing apparatus 2, and simultaneously therewith. This is called “cleaning simultaneous with developing”.
The mechanism of cleaning simultaneous with developing will be described here. This cleaning method is a method of collecting any toner residual on the photosensitive drum after transferring during the developing after the next process, i.e., in case of continuedly charging the photosensitive member to thereby form a latent image, and developing the latent image, by a defogging bias (a defogging potential difference V back which is the potential difference between a DC voltage applied to developing means and the surface potential of the photosensitive drum) (see Japanese Patent Application Laid-Open No. 10-307456).
According to this method generally called a cleanerless system, the untransferred toner (residual toner) is collected by the developing means and is reused for the next and subsequent processes and therefore, waste toner can be eliminated and cumbersomeness in maintenance can be decreased.
Also, because of being cleanerless, the advantage in space is great and the image forming apparatus can be greatly downsized.
A description will now be made of the cleaning of the charging member 3 and the transfer member (transfer roller) 5 in the cleanerless type image forming apparatus using the contact charging member (charging roller) 3.
Because of the contact charging member being used, the untransferred toner not transferred in the transferring part but left on the photosensitive drum 1 is once introduced into the charging member 3. Generally the toner is an insulative substance and therefore, if the toner is present on the charging member 3 in a great deal, it will cause defective charging. So, a discharging bias is applied to the charging member 3 to thereby return the untransferred toner having entered the charging member to the photosensitive drum 1. Thereafter, the untransferred toner having reached the developing means 2 is returned into the developing device by a collecting bias by the developing means 2.
Also, when a material having powder retaining capability like sponge is used as the transfer member 5, the toner present on the photosensitive drum 1 in the absence of the transferred member P is caught by the transfer member 5 and becomes the cause of the stains of the end portion of the paper which is the transferred member P. So, a discharging bias is applied to the transfer member 5 to thereby return the untransferred toner having entered it to the photosensitive drum 1. Thereafter, the toner discharged in the same manner as described above is returned into the developing device by the collecting bias by the developing means 2.
However, in the case of an image forming apparatus in which a discharging bias is applied to the charging member 3 and the transfer member 5 to thereby effect cleaning, the removal of the toner or powder particles adhering to the charging member 3 and the transfer member 5 has been incomplete in some cases. In such cases, when the image forming operation is repeated, image defects such as fog, an uneven image, light interception and the stain of the end portion of the paper occur. The causes of the occurrence of those image defects will be shown below.
1) Uneven Image
Generally the toner is an insulator and therefore, if the toner is present in the contact portion a between the charging member 3 and the photosensitive drum 1, the charging of the surface of the photosensitive drum will be interrupted. As a result, unevenness will occur to the toner image on the photosensitive drum and that portion will become an image defect called an uneven image. This uneven image is very liable to occur as the untransferred toner is accumulated on the charging member 3.
2) Fog
The toner introduced onto the charging member 3 causes the deterioration of the toner such as the extraneous additive of the toner being embedded or stripped off by the charging member being rotated, and along therewith, the charging capability of the toner is lowered. If such toner drops onto the photosensitive drum 1, even if a collecting bias is applied to the developing device, the toner will not be returned to the developing device, but will be intactly transferred onto the transferred member (such as paper) P. When this toner is present in other area than a latent image area, it becomes an image defect called fog. This fog toner is introduced into the transfer member 5 when paper or the like which is the transferred member P is absent in the transferring nip part d. As the fog toner increases, the toner in the transfer member 5 also increases in proportion to it, and the stain of the end portion of the paper becomes liable to occur.
3) Light Shielding (Light Interception)
This intercepts exposure when the toner or electroconductive powder is present in a great deal on the surface of the photosensitive roller after the charging of the charging roller. Therefore, this causes the image defect that a toner image is not formed at the intercepted position. When the toner introduced into the charging member is great in quantity, as described above, it causes an image defect.
The reason why cleaning becomes incomplete by the conventional cleaning method will now be described with reference to FIG. 13 of the accompanying drawings. FIG. 13 is a potential model view during the cleaning in the above-described image forming apparatus using a negative charging polarity toner.
In FIG. 13, Vn is the potential of a charging bias being applied to the charging roller, Vt is the potential of a transferring bias being applied to the transfer roller, and Vdt is the potential of the photosensitive drum between transferring and charging when the charging bias and the transferring bias are being applied. The dark potential Vd of the photosensitive drum during image forming is set to −600V.
Vn=−620 to −720V(AC voltage)
Vd=−600V
Vt=−1000V
Vdt=−700V
As shown, at the transferring position d, the toner on the transfer member 5 is discharged onto the photosensitive drum by −1000V being applied to the transfer member. At that time, the surface potential Vdt of the photosensitive drum becomes −700V. Thereafter, the potential Vdt intactly reaches the charging nip part a which is the contact portion between the charging member 3 and the photosensitive drum 1. If at the charging position a, the surface potential of the photosensitive drum is normal −600V which is the dark potential during image forming, the toner in the charging member 3 is moved to the photosensitive drum 1 by the voltage Vn being applied, but the surface potential of the photosensitive drum is Vdt and therefore, there is a case where the toner in the charging member 3 remains uncharged and conversely the toner discharged from the transfer member 5 is introduced into the charging member 3.
This phenomenon particularly affects the time of jam of a high coverage image print, and fog is very liable to occur after the jam. It is because a great deal of toner is present on the photosensitive drum 1 during jam and almost all of it is introduced into the charging member 3 and the transfer member 5.